A New Joint Element for the Modeling of Contact Interaction and Damage in SFR Fuel Assemblies
Résumé
The fuel assembly of a sodium fast nuclear reactor (SFR) is a hexagonal tube containing around 200 fuel pins made of steel. In order to avoid overheating and damage on the assembly, a helical steel wire is wound around each pin. This wire maintains a suitable distance between pins and ensures a proper mixing of the sodium. Nevertheless, the intense fast neutron irradiation induces a significant isotropic swelling and creeping of the pins. This progressively leads to contact closure, helical bowing of the pins, and a strong ovalisation of the cladding sections. At such an interaction level, there is a possibility of cladding damage due to thermal creep accumulation.
Modeling such a structure is a numerical challenge: around 10 000 contact areas, highly nonlinear material behavior laws, high mesh density needed for damage location. Thus, a simplified approach is developed: the two components are modeled by classical finite elements with a low mesh density: the hexagonal tube by shell elements, the fuel pins by beam elements. In addition, we have developed a new joint element dedicated to the contact interaction between components and the radial crushing of the pins.
This element, with only two nodes, connects the neutral fiber of neighboring beams (or the mid-surface of shells). It takes into account:
-the gap and the contact between two pins (or a pin and the tube),
-the thermal expansion and the irradiation swelling of the pin cross sections (including heterogeneity of temperature),
-the radius increase by creeping under internal pressure loading,
-the elastic rigidity of the pin crushing (locally pinching a short section of a tube),
-the contact force, altered by the two creeping behaviors, when pinching this 3D tube section,
-the counter ovalisation force due to internal pressure on the deformed shape of the pins,
-the hot point stress and strain tensors on the internal skin (strain concentration),
-the hot point damage accumulation due to thermal creeping only.
This connecting element has been implemented in the Cast3M finite element code (http://www-cast3m.cea.fr). Eventually, the model validation is made:
-on the joint element, and compared to detailed 3D solid calculations,
-on the global fuel assembly model, and compared to a few relevant assembly experiments, which were conducted in the PHENIX French SFR.
It shows good agreement to the measurements and a very significant CPU performance gain compared to 3D solid models.